Copolyester elastomers are multi-block copolymers which are mainly comprised of polyester recurring units and polyether recurring units. Copolyamide elastomers are multi-block copolymers which are mainly comprised of polyamide recurring units and polyester or polyether recurring units. Both copolyester elastomers and copolyamide elastomers have an appropriate flexibility.
However, these copolyester elastomers and copolyamide elastomers are too hard to use as an elastomer in the rubbery region, and have a poor flexibility and a poor strain recovery. To improve these properties, it is generally employed to enhance the content of soft segments in the copolymer elastomer. However, the enhancement of the soft segment content reduces the melting point which leads to narrowing of the high-temperature range at which the elastomer is usable. Instead of enhancement of the soft segment content, it has been proposed to incorporate a softening agent such as a plasticizer in the copolymer elastomers. The incorporation of a softening agent has a problem such that the incorporated softening agent tends to exude to the surface or evaporate during the use of the elastomers, which leads to restoration to the reduced flexibility.
To solve the above-mentioned problem, proposals have been made wherein a rubber is incorporated in the copolymer elastomers. For example, it has been proposed in Japanese Laid-open Patent Publication No. 1-266154 to incorporate an acrylic rubber having copolymerized therein 1 to 5% by weight of a reactive and curable monomer, in a copolyester elastomer to provide a crosslinked or uncrosslinked acrylic rubber-containing composition. The uncrosslinked composition has a poor compression set. The crosslinked composition is deteriorated especially when crosslinked with typical crosslinkers for an acrylic rubber, which are exemplified in the Japanese patent publication, such as a quaternary ammonium salt, a soap/a tertiary or quaternary amine, a red lead oxide/ethylene thiourea or a polyamine. Namely, the copolyester elastomer is degraded during milling or shaping of the composition or during the use of a shaped article, which leads to the reduction in performances of the shaped article.
In Japanese Laid-open Patent Publication (JPA) No. 1-306447, a composition is described which is comprised of a thermoplastic polyester resin such as polyethylene terephthalate or polybutylene terephthalate and an acrylic rubber crosslinked through a covalent bond. The crosslinked acrylic rubber is an acrylic rubber prepared by crosslinking a polyacrylate having an acid group, a hydroxyl group or an epoxy group as the crosslinking site with a polyamine, a polyisocyanate or a polyepoxide through a covalent bond. As the composition contains a rigid polyester resin, the content of the rubbery component must be high for imparting a satisfactory compression set to the composition, with the result of reduction in fluidity of the composition in the step of shaping.
Although a rubber composition comprising a polyetherester elastomer and an ethylene-acrylate copolymer rubber crosslinked with an isocyanate compound is described as one example of the composition in JPA 1-306447, this reference is silent on crosslinking system of an epoxy group in a rubber composition of a copolyester elastomer and an epoxy group-containing methacrylate or acrylate copolymer rubber.
As suggested in the above-described references, crosslinked rubber compositions comprising a copolyester elastomer having an acrylic rubber crosslinked and dispersed in the compositions exhibit improved flexibility and compression set resistance. But, a satisfactory crosslinking system of an acrylic rubber in a copolyester elastomer has not been found in the crosslinked rubber compositions. Especially, a problem arises in that a copolyester elastomer tends to be readily deteriorated in a molten state by a crosslinker usually used for an acrylic rubber.